yes, the prince of tennis series is really popular in japan. and the previous awesome sales of the red and blue GBA versions didn't hurt it either. part of the reason is the characters are drawn to be attractive to girls as well, kind of like how gundam seed is done.
anyway, yeah i thought it was kindof excessive to have close to full voice for this, but hey, they have an anime series, and radio dramas, might as well have the voice actors do a game as well.